This invention relates to the field of materials, and particularly to the field of metal matrix structural composites.
Structural composites are two (or more) phase materials which have bulk mechanical and physical properties derived from the separate properties of the individual phases forming the composite. By combining two materials, such as a ductile aluminum or titanium alloy and a high modulus silicon carbide powder or whisker, a composite can be formed which has some of the ductility and fabricability of the metal alloy combined with a modulus of elasticity which is intermediate to that of the metal alloy and the silicon carbide.
There are several methods used to combine two materials into a single composite, such as infiltrating a fibrous phase with a liquid metal, mixing different phases together as powders and then consolidating the mixture by powder metallurgy techniques, electroforming, plasma spray bonding, and press diffusion bonding. There are numerous problems encountered with each of these methods which severely limit the quality and economy of the resulting composite.
A common method for fabricating aluminum-matrix composites is by powder metallurgy techniques. The first step is to mix pre-alloyed aluminum and powders (or whiskers) in appropriate proportions and disperse then uniformly by mechanical mixing. Evacuation is performed to remove all included gases and then hot consolidation is performed, generally above the solidus temperature. Subsequently, hot extrusion or shaping follows. While reasonably good strength and modulus values are exhibited by these materials, the two main drawbacks of such processing technique are (1) poor ductility and fracture toughness of the final product, and (2) poor formability, i.e., lack of the capability of shaping them in all but the simplest shapes by compression (forging) processes. It is likely that the main reason for the poor ductility of aluminum matrix composites made from powder metals is the presence of oxide film surrounding the powder particles. Furthermore, the fine powders make it extremely difficult to completely remove gas from the billet, and gas porosity can remain. This could cause serious problems with the evolution of gases during heat treatment or welding of these aluminum-matrix composites.
This invention utilizes press diffusion bonding, i.e. solid-state joining, to combine the two materials. In the conventional press diffusion bonding method, alternate layers of continuous reinforcement and metal foil are arranged in a stack. The stack is heated to a temperature suitable for diffusion bonding, and sufficient pressure is applied to cause the metal to flow between the continuous reinforcement and diffusion bond to itself as well as to the inter-layered materials. It is difficult to obtain flow and intimate contact of the solid metal with all the surface of each continuous reinforcement and with mating surfaces of the metal foil. This difficulty would appear to be even greater when a discontinuous reinforcement is used. Consequently, powder metallurgy techniques have been used rather than press diffusion bonding when it was desired to incorporate discontinuous reinforcement in a composite.
The pressure requirement of the diffusion bonding process is a major concern because of the relationship between press capacity and plan area size of the composite which can be produced. High pressure during consolidation also tends to damage reinforcing fibers. Another concern is the constraint on the maximum pressure due to the ceramic platens normally used and stresses induced in the tooling. Thus, the general approach is to utilize as high a temperature as possible in order to reduce the pressure requirements. However, high temperature increases the reactivity between the reinforcement and metal matrix, and reduces the strength of the finished composite.
High temperature also causes oxidation of the materials being consolidated. Consequently, metal alloy matrix composites are generally fabricated in vacuum using a retort. Even when vacuum is used, the transverse mechanical properties are inferior because of poor quality bonds resulting from inadequate deformation and breakdown of the tenacious oxide layer on the metal foil.